1. Field of the Invention
The present invention relates to a jelly-roll type electrode assembly and a secondary battery including the same, and particularly to, a jelly-roll type electrode assembly in which short-circuiting between electrodes can be prevented, and a secondary battery including the same
2. Description of the Related Art
In general, secondary batteries, which are distinguished from primary batteries by their ability to be repeatedly charged and discharged, have been widely used in cellular phones, notebook computers, camcorders, and other portable electronic devices. Lithium secondary batteries having an operating voltage of 3.6V or greater, which is three times higher than nickel-cadmium (Ni—Cd) batteries, have become popular as a power source for various kinds of electronic equipment and nickel-hydrogen batteries and, are frequently used because of their high energy density per unit of weight.
Such lithium secondary batteries mostly use a lithium oxide as a positive active material and a carbonaceous material as a negative active material. Lithium secondary batteries can be classified into liquid electrolyte batteries, also known as lithium ion batteries, and polymer electrolyte batteries, also known as lithium polymer batteries, according to the type of electrolyte used. Lithium secondary batteries are manufactured in various shapes, typically, in cylindrical, rectangular, or pouch forms.
A second battery uses a jelly-roll type electrode assembly manufactured by applying an active material onto a substrate, drying the substrate, pressing it using a roller, and severing it into a positive electrode strip and a negative electrode strip, interposing a separator between the positive and negative electrode strips to make a multi-layer structure, and rolling the multi-layer structure in a jelly-roll form. A cylindrical battery is made by placing such an electrode assembly into a cylindrical can, injecting an electrolyte into the cylindrical can, and sealing the cylindrical can. A rectangular battery is made by applying a pressure to such an electrode assembly so that it has plane surfaces, and then, placing the electrode assembly into a rectangular can.
In such an electrode assembly, a positive electrode tab and a negative electrode tap are drawn out from the positive electrode strip and the negative electrode strip, respectively. In particular, one of the positive and negative electrode tabs is drawn out upward from its related electrode strip and electrically connected to a cap assembly for sealing the can. The other electrode strip is drawn out downward from its related electrode strip and electrically connected to a projection formed at the bottom of the can. In the case of the rectangular secondary battery, both the positive and negative electrode tabs may be drawn out upward from their electrode strips.
As described above, a conventional cylindrical battery includes an electrode assembly made by interposing only a separator between a major electrode strip and an auxiliary electrode strip to make a multi-layer structure and rolling the multi-layer structure. The separator is, however, prone to be broken at portions of the positive and negative electrode strips welded together with electrode tabs, thus causing electrical short-circuiting. An electrode tab enables movement of electric charges from a battery to the outside, and therefore, when electric charges are concentrated on the electrode tab, the electrode tab is heated. As a result, an electrode strip attached with the heated electrode tab is also heated, thus resulting in breaking of the separator. Then, although the electrode strip attached with the electrode tab is separated via the separator from the other electrode strip of different polarity, the breaking of the separator causes electrical short-circuiting between the heated electrode strip and the other electrode strip.
Also, when a defect, such as a burr, forms at an end of an electrode tab during manufacture of the electrode tab, the burr damages the separator, especially a thin separator, thus causing an electrical short-circuiting via a damaged portion of the separator.
Japanese Patent Publication No. 11-273660 has suggested a battery electrode assembly in which a portion of an electrode strip welded together with an electrode tab is coated with a polymer material, thereby preventing a separator from being damaged due to the formation of a burr or other defect of the electrode tab. However, in this case, since a conventional battery manufacturing process further requires a polymer coating process, the manufacture process is more complicated and manufacturing costs are increased.
Japanese Patent Publication No. 4-109551 discloses a jelly-roll type battery electrode assembly in which an electrode tab is bent inward more than the rolled battery electrode assembly and a bent portion of the electrode tab is located at a center portion of an inner diameter of the rolled electrode assembly. However, it is very difficult to position the electrode tab at the center of the rolled electrode assembly. Even if the electrode is positioned at the center, it is difficult to insert a welding rod into the electrode assembly due to the presence of the electrode tab during a subsequent process of welding an electrode tab together with a base plane of a can.
U.S. Pat. No. 5,508,122 discloses a secondary battery with a spiral electrode unit. In the spiral electrode unit, same-polarity regions of an electrode strip attached with a lead toward a center portion of the spiral electrode unit are positioned via a separator on both sides of exposed regions of electrode core material, and a lead attached to the other electrode strip is positioned at the outmost winding of the spiral electrode unit. Accordingly, even if the separator is damaged, electrical short-circuiting can be prevented.
However, in the spiral electrode unit, the exposed regions must be long enough to position the same-polarity regions of the electrode strip via a separator on both sides of the exposed regions, and a starting point of rolling the other electrode strip is later than that of rolling the electrode strip. Thus, the secondary battery includes many portions of electrode strips unnecessary for battery reaction, thereby increasing manufacturing costs. If the secondary battery is rectangular shaped, that is, when a length of the battery in circumference is long, this problem becomes more serious.